1. Field of the Invention
The present invention relates to an electron emitting element and a method for producing the same.
2. Description of the Related Art
A Spindt-type electrode and a carbon nanotube (CNT) electrode have been known as conventional electron emitting elements. Applications of such electron emitting elements to, for example, the field of FED (Field Emission Display) have been expected. Such electron emitting elements are caused to emit electrons by tunnel effect resulting from an intense electric field of approximately 1 GV/m that is formed by application of a voltage to a pointed section.
However, these two types of electron emitting elements have the intense electric field in the vicinity of a surface of an electron emitting section. Accordingly, electrons emitted obtain a large amount of energy due to the electric field to be more likely to ionize gas molecules. Cations generated due to the ionization of gas molecules are accelerated toward and collide with a surface of the electron emitting element due to the intense electric field. This causes a problem of breakdown of the electron emitting element due to sputtering.
Further, ozone is generated before ions are generated, because oxygen in the atmosphere has dissociation energy that is lower than ionization energy. Ozone is harmful to human bodies and oxidizes various substances because of its strong oxidizing power. This causes a problem in that members around the electron emitting element are damaged. In order to deal with this problem, expensive materials having resistance to ozone should be used for the members around the element.
Meanwhile, against the problems, MIM (Metal Insulator Metal) type and MIS (Metal Insulator Semiconductor) type electron emitting elements have been known as means for preventing breakdown of the element due to sputtering and lessening generation of ozone. These electron emitting elements are surface-emission-type electron emitting elements, each of which accelerate electrons by utilizing quantum size effect and an intense electric field in the electron emitting element so that electrons are emitted from a flat surface of the element. Accordingly, these electron emitting elements do not require an intense electric field outside the elements, because the electrons accelerated in an electron acceleration layer in the elements are emitted to the outside. The MIM type and MIS type electron emitting elements can therefore overcome the problem of breakdown of the element by sputtering due to ionization of gas molecules and the problem of ozone generation, which are likely in the Spindt-type, CNT type and BN type electron emitting elements.
In addition, against the problems, there has been developed an electron emitting element that is capable of stable electron emission in the atmosphere and inhibited from generating harmful substances such as ozone and NOx when emitting electrons. For example, there has been known an electron emitting element including, between electrodes, an electron acceleration layer containing: conductive fine particles composed of an electrical conductor and having a strong antioxidative effect; and an insulating material having a size larger than the size of the conductive fine particles (see for example, Japanese Unexamined Patent Publication No. 2009-146891).
In some cases, an electron emitting element cannot operate continuously, while it can lessen harmful substances such as ozone and NOx to be generated upon electron emission. Specifically, a nonuniform electric field may be formed in a part of an electrode, and the electric field may be concentrated in a part of an electron acceleration layer. In this case, if the electron emitting element continuously operates with the concentration of electric field being left as is and application of current goes on, the current is concentrated in the part through which the current flows more easily. This sometimes leads to dielectric breakdown and, as a result, disrupts application of a voltage to the element. It has been therefore desired to develop an electron emitting element that can operate continuously over a long period of time.